Optogenetics and scientific imaging applications require high brightness, incoherent light source of certain wavelengths and bandwidth, mainly in visible spectrum. Current solutions like laser pumped plasma light source and white light LED exhibit very high output optical power, but limited brightness due to their relatively large etendue. Supercontinuum lasers have quite high brightness, but most of its output optical power resides in infrared (IR) spectrum, not that useful for the applications aforementioned.
Phosphor based light source provides another possibility to meet this need: Crystalline phosphor material demonstrates suitable emission light wavelength and bandwidth for most applications, and high pumping power sustainability, which means the phosphor converts high brightness incident light of relatively short wavelength into high intensity emission light of longer wavelength.
During the development of broadband phosphor light source, the applicant of the present invention has found that it is difficult to improve the light emission brightness of the phosphor. There are several reasons behind it: (1) Without proper pumping light confinement, the phosphor emission exists in a large volume of the phosphor. So the phosphor emission power concentration is relatively low, even if the total emission power is high. Simply focusing the incident light onto the phosphor doesn't solve the problem, for the phosphor would get overheat and its light conversion efficiency drops down significantly. (2) The emission of phosphor is quasi-isotropic, even if the incident pumping light is in a narrow angle. Only a small portion of the phosphor emission light is collected into the out-coupling fiber if the optical setup is not well-designed. These two problems have to be solved before high output optical power through the output fiber or fiber bundle can be achieved.
The optical design of current phosphor light sources, however, is yet to be optimized. There has been some effort paid to design phosphor based light source.
For example, documents U.S. Pat. No. 8,709,283 B2, U.S. Pat. No. 8,770,773 B2 and US2011/0175510 A1 focus on LED pumped phosphor light source. In these designs the LED emits short wavelength light, such as blue light or UV light. The phosphor is made as a cover layer, a plate or a tube. The LED emission light is absorbed by the phosphor, which emits light of longer wavelength in turn. These designs, however, do not involve optical setups to focus LED emission light into a small domain of the phosphor. Therefore, the light conversion exists in a relatively large volume of space, equaling the size of the phosphor itself. Due to the large emission volume, the phosphor emission brightness is relatively low, which limits its application in optogenetics and scientific imaging.
Disclosed in U.S. Pat. No. 8,709,283 B2 and U.S. Pat. No. 8,770,773 B2 are two typical phosphor light source designs using LED and LED array as the pumping light source. These designs have not introduced any special optical design to improve the phosphor emission brightness, yet. This is because that these product designs are for general illumination applications, and high brightness is not a primary goal for their designs.
M. Cantore et al. has paid attention to the luminous efficacy of laser pumped phosphor light source in their publication “High luminous flux from single crystal phosphor-converted laser-based white lighting system” (Opt. Expr. Vol. 24, No. 2, January 2016). But without special light concentration design, their maximum luminous flux at 14W laser pumping power is 1100 lm, isotropically emitted.
In the documents US2014/0253882 A1 and OSRAM light source ITOS PHASER, there are well-designed light focusing setups to concentrate the incident light onto the phosphor, improving the phosphor emission brightness. However, multiple laser light sources are used in both designs, which make it difficult to confine the laser spot to a minimal size on the phosphor surface. The OSRAM PHASER achieves 2100 lm luminous flux, almost twice higher than M. Cantore's record, but the brightness (luminous flux per unit area of fiber core) is only ˜200 lm/mm2, still not high enough for scientific applications.
Further improvement of phosphor emission brightness requires a smaller focusing spot of pumping light source onto the phosphor (such as employing a single high power laser rather than an array of low power lasers). However, for such a high concentration of pumping light power, heat dissipation of phosphor becomes a challenge. Encarnacion Villora et al. has reported the overheat phenomena of phosphor material in the paper “Single-Crystal Phosphors for High-Brightness White LEDS and LDs” (J. of Japanese Assoc. for Crystal growth, Vol. 42, No. 2, 2015). Their analysis concludes that single crystal phosphor sustains higher temperature than conventional ceramic phosphor, but optimized active cooling design is still necessary if high concentration laser pumping is involved.
In another document US2014/0253882 A1, a lens group is employed to focus input pumping laser light onto a small area of the phosphor. However, this setup involves a wheel coated with different kinds of phosphor, so as to acquire different light emission spectrums when different phosphors are pumped in sequence. The price they pay is that the emission spectrum is not constant and stable, but keeps flipping at a pre-set frequency. The emission brightness at a certain wavelength is sacrificed, too, as each single domain of phosphor is pumped for less than ⅓ of total operation time (as the red, green/cyan, yellow and blue sections are pumped one after another). This design principle is necessary for commercial display applications, but it may not be suitable for the scientific applications aforementioned, which does not require full-color imaging, but has a higher requirement on brightness and stability of phosphor emission.
Besides, this phosphor wheel design does not involve any heat sink or other active cooling apparatus, and relies on thermal emission into air and conduction into the wheel base. This limits the maximum pumping laser intensity applied on the phosphor, and the maximum phosphor emission brightness in turn.
Therefore, a new phosphor light source with optimized optical setup is to be proposed to address the issues above.